1. Field of the Invention
The present invention relates to a current limiter circuit for protecting input circuits. More particularly, the present invention relates to a current limiter circuit for protecting input circuits from excessive over-voltage conditions and excessive input currents, while providing low distortion for small-signal input voltages.
2. Description of the Related Art
Input circuits appear in a wide variety of applications, including instrumentation devices such as Digital Multimeters (DMMs), oscilloscopes, spectrum analyzers and general purpose data acquisition equipment. Typically, input protection is required for preventing input circuits from destruction caused by over-voltage conditions.
In many cases, a simple arrangement of diode clamps are utilized for limiting the input voltage to the internal power supplies of the circuit. Such an arrangement, however, creates a condition in which excessive current may be injected externally through the clamp diodes.
FIG. 1 shows a schematic block diagram of a typical input circuit 100 having a conventional current limiter 101 for limiting excessive current. Input circuit 100 includes an input resistor R1, a current limiter device 101, two clamp diodes D1 and D2, and an amplifier A1. An input signal input at IN is applied to resistor R1. The input signal is coupled through current limiter device 101 to the input of amplifier A1. Current limiter device 101 is depicted in FIG. 1 as a resistor. The anode of clamp diode D1 is coupled to the input of amplifier A1. The cathode of clamp diode D1 is coupled to supply voltage VCC. The cathode of clamp diode D2 is coupled to the input of amplifier A1. The anode of clamp diode D2 is coupled to supply voltage VEE. Clamp diodes D1 and D2 limit the input voltage that can be applied to the input to amplifier A1 to about supply voltages VCC and VEE. Current input device 101 limits that amount of current that can be supplied externally to clamp diodes D1 and D2 and to the input of amplifier A1 when the externally applied input voltage exceeds the clamping voltages of VCC and VEE.
FIGS. 2A–2C depict circuit components that are conventionally used as current limiting devices. For example, FIG. 2A depicts a resistor 201. FIG. 2B depicts a Positive Temperature Coefficient (PTC) thermistor 202. FIG. 2C depicts a light bulb 203.
Another example of a conventional input protection circuit is disclosed by U.S. Pat. No. 5,742,463 to Harris. According to Harris, such an input protection circuit includes at least two depletion-mode field effect transistors, and can provide unipolar or bipolar operation, thereby protecting an input circuit from both positive-going and negative-going voltage transients.
The goals of an ideal current limiter include the capability to prevent destruction of input components, including the limiter itself. Input current for such an ideal current limiter should be limited based on the maximum expected input voltage. The ideal current limiter should also provide a low-noise, highly-linear, low-value impedance for normal, small-voltage operating conditions, while providing a high impedance for large voltages. Thus, the impedance state of an ideal current limiter must change based on the applied voltage. Additionally, both the inrush transient current and static power dissipation of an ideal current limiter should be minimized for preventing failure of any components.
What is needed is yet a better technique for limiting overload current for preventing destruction of clamp diodes and input circuitry.